The anodization of aluminium and aluminium alloys leads to the formation of an oxide layer which has excellent adherence to the metal, a high level of flexibility and hardness, but, because of its porous structure, a weak resistance to external chemical agents. From this weakness derives the industrial treatment defined as "sealing", which gives the oxide a satisfactory resistance to external agents so as to render the manufactured products treated in this way useable for the most varied purposes, even in severe conditions. The sealing methods which have evolved from the beginning of the practice of anodic oxidation of aluminium and aluminium alloys are all different, but they mostly consist in the immersion of the anodized piece in demineralized water or in an aqueous solution containing nickel or cobalt salts at a temperature of about 100.degree. C. for periods of time varying from half an hour to an hour, depending on the thickness of the oxide.
The generally accepted opinion is that in the sealing treatment the oxide is transformed into bohemite; this transformation occurs above 80.degree. C. and its efficiency increases with the increase in the temperature. The theories on sealing are also different from the first simplistic idea that the sealing provokes the closing of the pores. It is now thought that the action is more complex, including a partial restriction of the pore and the formation of waterproof gel layers at the pore's base. The action of nickel and cobalt salts and other mineral salts is explained by their tendency to precipitate hydroxides in the pH conditions of the means used.
The prior art processes, while giving satisfactory results, do not however represent the maximum. These types of sealing, even when carried out in the best conditions, do not render the article resistant to the various corrosive elements in the environment, even the bland ones, and particularly not in the case of alkaline elements such as the lime for plaster and water soluble colours. Alkaline resistance is very important given the extensive use of aluminium oxide in the building industry.
Further, the costs of the prior art processes are high, given the remarkable consumption of energy to keep the temperature of the bath near boiling point; the cost of the steam process which is often used as an alternative is equally high.
It is therefore natural that for some time the industry has been trying to perfect anodic oxide treatment processes which will at the same time give better protection at lower cost.